Aluminum oxynitride (AlON) is a ceramic material comprising aluminum (Al), oxygen (O), and nitrogen (N) atoms. Aluminum oxynitride materials have been reported to exhibit a fracture strength of between about 450 megapascals (MPa) and about 500 megapascals (MPa), a fracture toughness of between about 2.6 MPa(m1/2) and about 2.9 MPa(m1/2), and a hardness of between about 15 gigapascals (GPa) and about 20 gigapascals (GPa). See e.g., H. X. Willems et al., Mechanical properties of γ-aluminum oxynitride, Journal of Materials Science 28, 6185-6189 (1993). Furthermore, aluminum oxynitride may be formed as a polycrystalline material, and may be formed to be at least substantially transparent to visible light.
Aluminum oxynitride has been used in a wide variety of applications for its physical properties, in additional applications for its transparency to visible light, and in yet further applications for both its physical properties and transparency. For example, due to its physical properties and its transparency to visible light, aluminum oxynitride is being investigated as a possible armor material.
Aluminum oxynitride may be manufactured using what is often referred to as a reaction sintering process, in which a mixture of powdered aluminum oxide (also referred to as “alumina”) and powdered aluminum nitride is heated and reacted to form aluminum oxynitride material. In such methods, powdered aluminum oxide may be mixed with powdered aluminum nitride such that the resulting powder mixture has a composition of between about sixty (60) and eighty (80) mole percent (mol %) aluminum oxide, the remainder being aluminum nitride. The powdered aluminum oxide and powdered aluminum nitride each may have an average particle size of less than about one-hundred microns (100 μm). The powder mixture of aluminum oxide and aluminum nitride may be milled (e.g., in a ball mill) and subsequently calcined to form powdered aluminum oxynitride material. For example, the powder mixture may be calcined in a crucible (which may be formed of aluminum oxide) at a temperature of between about 1600° C. and about 1750° C. for about four (4) hours with a stagnant atmosphere of nitrogen gas (N2) at a pressure range of between about zero (0) and about five (5) pounds per square inch gauge (psig).
In additional methods, aluminum oxynitride powder may be produced by heating aluminum oxide in the presence of nitrogen and a reducing agent. The reducing agent causes some of the aluminum oxide to react with the nitrogen to form aluminum nitride, which then reacts with the remaining aluminum oxide to form aluminum oxynitride powder. Common reducing agents include, for example, aluminum and carbon.
In the methods described above, agglomerates of aluminum oxynitride particles may form during the formation of the aluminum oxynitride particles themselves, and such agglomerates must be removed from the bulk powdered aluminum oxynitride material. Furthermore, the aluminum oxynitride particles often need to be ground to a finer average particle size prior to using the particles to form a three-dimensional solid aluminum oxynitride body from the particles. Such grinding processes may be difficult due to the relatively high hardness of the aluminum oxynitride material.
The resulting aluminum oxynitride powder then may be processed to form a three-dimensional solid body of aluminum oxynitride material. For example, the aluminum oxynitride powder may be used to form a green body, which then may be sintered to a desired final density. Such a green body may be formed by axially or isostatically pressing the aluminum oxynitride powder in a mold or die. Casting techniques (e.g., slip casting or tape casting) also may be used to form a green body from aluminum oxynitride powder.
After forming a green body, the green body may be sintered to a desired final density. The solidus line of the phase diagram for aluminum oxynitride material is located at approximately 2140° C. Solid-state sintering may be conducted at a temperature of between about 1900° C. and about 2140° C. for between about twenty (20) hours and about one-hundred (100) hours in a nitrogen gas atmosphere at a pressure range of between about zero (0) and about five (5) pounds per square inch gauge (psig).
The green body may be sintered with or without applied pressure. It has been observed, however, that sintering without applied pressure can result in porosity in the resulting sintered aluminum oxynitride body, which may cause a reduction in the transparency of the aluminum oxynitride body. Thus, standard pressureless sintering is generally considered to be incapable of attaining transparent aluminum oxynitride bodies.
Current methods for attaining sufficiently transparent aluminum oxynitride bodies employ the use of dopants in the green body and pressured sintering techniques, such as hot pressing (e.g., hot isostatic pressing) techniques. Hot pressing involves applying relatively high pressures to a body as the body is sintered. The applied pressure assists in the densification process and results in a significant reduction in the porosity of the fully sintered body. Furthermore, it has been observed that the inclusion of dopants (e.g., lanthanum (La), yttrium (Y), magnesium oxide (MgO) (often referred to as “magnesia”), and/or yttrium oxide (Y2O3) (often referred to as “yttria”)) in the green body may further enhance the densification process during sintering, which may result in a further reduction in the porosity of the fully sintered body. It is believed that the dopants assist densification by forming a small amount of liquid phase during sintering that modified grain growth in a beneficial manner. In small amounts, the dopants will dissolve into the resulting aluminum oxynitride material and will not form detrimental phases therein.